Multi-layer hard mask structure for etching deep trench in substrate

ABSTRACT

A method for etching a deep trench in a substrate. A multi-layer hard mask structure is formed overlying the substrate, which includes a first hard mask layer and at least one second hard mask layer disposed thereon. The first hard mask layer is composed of a first boro-silicate glass (BSG) layer and an overlying first undoped silicon glass (USG) layer and the second is composed of a second BSG layer and an overlying second USG layer. A polysilicon layer is formed overlying the multi-layer hard mask structure and then etched to form an opening therein. The multi-layer hard mask structure and the underlying substrate under the opening are successively etched to simultaneously form the deep trench in the substrate and remove the polysilicon layer. The multi-layer hard mask structure is removed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a hard mask structure foretching a deep trench etch in a substrate, and more particularly, to aboro-silicate glass (BSG) containing multi-layer hard mask structure foretching a deep trench in a substrate.

2. Description of the Related Art

Dynamic random access memory (DRAM) is an important semiconductor devicein the information and electronics industry. Most DRAMs presently haveone access transistor and one storage capacitor in one DRAM cell. Withincreased integration, however, 3-D capacitors, such as deep trenchcapacitors, have become necessary. Typically, 3-D capacitors aredisposed in a deep trench formed by etching a silicon substrate.Thereafter, an access transistor is formed overlying the deep trenchcapacitor to complete a deep trench DRAM cell.

In the formation of integrated circuits, it is often necessary to etch asilicon substrate to form a trench therein. In particular, the trendtowards packing more memory cells into a given chip area has led to thedevelopment of trench DRAM cells which require deep and narrow aperturesin the silicon substrate.

FIGS. 1 a to 1 c are cross-sections showing a conventional method foretching a deep trench in a silicon substrate. First, in FIG. 1 a, asilicon substrate 100 is provided. A pad dielectric layer comprising athin silicon oxide layer 102 and an overlying silicon nitride layer 104is formed in the silicon substrate 100. Next, a hard mask layer 106,such as a BSG layer, is formed overlying the pad dielectric layer 102and 104 for etching the subsequent deep trench. Thereafter, an optionalannealing process is performed on the hard mask layer 106. A photoresistpattern layer 108 is formed on the hard mask layer 106 and has anopening 110 to expose the region for etching deep trench.

Next, in FIG. 1 b, the hard mask layer 106 and the underlying paddielectric layer 102 and 104 under the opening 110 are successivelyetched to form an opening 112 therein, thus exposing the surface of thesilicon substrate 100. Next, the photoresist pattern layer 108 isremoved.

Finally, in FIG. 1C, the exposed surface of the substrate 100 is etchedusing the hard mask layer 106 as an etching mask to form a deep trench114 in the substrate 100. Meanwhile, a portion of the hard mask layer106 is consumed by etching.

After the deep trench 114 is formed, the hard mask layer 106 isnecessary to be removed. The hard mask layer 106 can be removed by vaporhydrofluoric acid (VHF), HF solution, or a buffer oxide etching (BOE)solution. However, after the hard mask layer 106 composed of BSG isannealed, the boron atoms diffuse upwardly. As a result, poor boron atomconcentration uniformity is obtained and a practically undoped region(not shown) is formed near the bottom of the hard mask layer 106. Sincesuch an undoped region is difficult to remove by VHF, a remaining hardmask layer 106 a is formed on the pad dielectric layer 102 and 104, asshown in FIG. 2 a. Additionally, if the hard mask layer 106 is removedby HF solution or BOE solution, there is no remaining hard mask layer.However, the underlying thin silicon oxide layer 102 is undercut, asshown by the arrows in FIG. 2 b. As a result, the overlying siliconnitride layer 104 is subject to peeling, thus reducing device yield.

Although no hard mask layer remains without previous annealing of thehard mask layer, various depths and critical dimensions (CDs) resultfrom subsequent deep trench formation in the substrate.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a novelmethod for etching a deep trench in a substrate, which employs amulti-layer hard mask structure for deep trench etching, instead of theconventional single boro-silicate glass (BSG) layer as the hard mask,and the multi-layer hard mask structure is then annealed, therebyimproving boron atom concentration uniformity in the hard mask andpreventing deep trenches from having various depths and criticaldimensions (CDs).

Another object of the present invention is to provide a novelmulti-layer hard mask structure, which has an undoped silicate glass(USG) layer disposed between each BSG layer to serve as a diffusionbarrier layer, thereby prevent upward diffusion of boron atoms in thehard mask thermal processes, preventing formation of residue subsequentto removal of the hard mask.

Yet another object of the present invention is to provide a novel methodfor etching a deep trench in a substrate, which uses a multi-layer hardmask structure and a BSG layer with higher doping concentration isformed in the bottom of the multi-layer hard mask structure, therebymaintaining boron atom concentration uniformity in the multi-layer hardmask structure.

According to the object of the invention, a multi-layer hard maskstructure is provided, which includes a first hard mask layer and atleast one second hard mask layer. The first hard mask layer is composedof a first boro-silicate glass layer and an overlying first undopedsilicon glass layer. The second hard mask layer is disposed on the firsthard mask layer, which is composed of a second boro-silicate glass andan overlying second undoped silicon glass layer.

Moreover, the doping concentration of the first boro-silicate glasslayer is about 4×10¹⁷ to 8×10¹⁷ atom/cm² and substantially equal to thatof the second boro-silicate glass layer.

In addition, the doping concentration of the first boro-silicate glasslayer can be higher than that of the second boro-silicate glass layer,wherein the doping concentration of the first boro-silicate glass layeris about 4×10¹⁷ to 8×10¹⁷ atom/cm² and that of the second boro-silicateglass layer is about 1×10¹⁷ to 5×10 ¹⁷ atom/cm².

Moreover, the first boro-silicate glass layer has a thickness of about0.3 μm. The first undoped silicon glass layer has a thickness of about100 to 400 Å and the second undoped silicon glass layer has a thicknessof about 100 to 400 Å.

Still according to the object of the invention, a method for etching adeep trench in a substrate is provided. A multi-layer hard maskstructure is formed overlying the substrate, which includes a first hardmask layer and at least one second hard mask layer disposed thereon. Thefirst hard mask layer is composed of a first boro-silicate glass layerand an overlying first undoped silicon glass layer and the second hardmask layer is composed of a second boro-silicate glass layer and anoverlying second undoped silicon glass layer. Next, a polysilicon layeris formed overlying the multi-layer hard mask structure and then etchedto form an opening therein and expose a portion of the multi-layer hardmask structure. Next, the multi-layer hard mask structure and theunderlying substrate under the opening are successively etched tosimultaneously form the deep trench in the substrate and remove thepolysilicon layer. Finally, the multi-layer hard mask structure isremoved. Moreover, the multi-layer hard mask structure can be annealedat 550 to 600° C. for 15 to 25 min before the polysilicon layer isformed thereon.

Moreover, the doping concentration of the first boro-silicate glasslayer is about 4×10¹⁷ to 8×10¹⁷ atom/cm² and substantially equal to thatof the second boro-silicate glass layer.

In addition, the doping concentration of the first boro-silicate glasslayer can be higher than that of the second boro-silicate glass layer,wherein the doping concentration of the first boro-silicate glass layeris about 4×10¹⁷ to 8×10¹⁷ atom/cm² and that of the second boro-silicateglass layer is about 1×10⁷ to 5×10¹⁷ atom/cm².

Moreover, the first boro-silicate glass layer has a thickness of about0.3 μm. The first undoped silicon glass layer has a thickness of about100 to 400 Å and the second undoped silicon glass layer has a thicknessof about 100 to 400 Å. The polysilicon layer has a thickness of about0.2 to 0.3 μm.

DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawings,given by way of illustration only and thus not intended to be limitativeof the present invention.

FIGS. 1 a to 1 c are cross-sections showing a conventional method foretching a deep trench in a silicon substrate.

FIG. 2 a is a cross-section showing a remaining hard mask layer formedover a deep trench with residue thereon of the prior art.

FIG. 2 b is a cross-section showing an undercut pad dielectric layerformed over a deep trench of the prior art.

FIGS. 3 a to 3 d are cross-sections showing a method for etching a deeptrench in a substrate according to the invention.

FIG. 4 is a cross-section showing a multi-layer hard mask structureaccording to one embodiment of the invention.

FIG. 5 is a cross-section showing a multi-layer hard mask structureaccording to another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 3 a to 3 d are cross-sections showing a method for etching a deeptrench in a substrate according to the invention.

First, in FIG. 3 a, a substrate 200, such as a silicon substrate, isprovided. A pad dielectric layer is formed on the substrate 200, whichcan be composed of a pad oxide layer 202 and a thicker silicon nitridelayer 204. In this invention, the pad oxide layer 202 can be formed bythermal oxidation. The silicon nitride layer 204 overlying the pad oxidelayer 202 can be formed by low-pressure CVD (LPCVD).

Next, a multi-layer hard mask structure 206 is formed overlying the paddielectric layer 202 and 204. In order to simplify the diagram, only aflat substrate is shown.

Next, in FIG. 4, a cross-section of the multi-layer hard mask structure206 according to an embodiment of the invention is shown. Themulti-layer hard mask structure 206 includes a first hard mask layer 205and a second hard mask layer 207 disposed thereon. In the invention, thefirst hard mask layer 205 is composed of a boro-silicate glass (BSG)layer 10 and an overlying undoped silicon glass (USG) layer 20. Theboro-silicate glass layer 10 has a thickness of about 0.3 μm and theundoped silicon glass layer 20 has a thickness of about 100 to 400 Å.The undoped silicon glass layer 20 is used as a diffusion barrier layer.Accordingly, the boron atoms in the boro-silicate glass layer 10 arediffused in the overlying undoped silicon glass layer 20 during heattreatment and prevent upward diffusion.

The second hard mask layer 207 is also composed of a boro-silicate glasslayer 30 and an overlying undoped silicon glass layer 40. Theboro-silicate glass layer 30 has a thickness of about 1.2 to 1.4 μm andthe undoped silicon glass layer 40 has a thickness of about 100 to 400Å. Here, the doping concentration of the boro-silicate glass layer 10 isabout 4×10¹⁷ to 8×10¹⁷ atom/cm² and substantially equal to that of thesecond boro-silicate glass layer 30. In the invention, the undopedsilicon glass layers 20 and 40 are used as diffusion barrier layers forthe double hard mask layers 205 and 207, respectively, to prevent apractically undoped region from forming near the bottom of the hard maskstructure 206 due to the poor doping concentration uniformity after heattreatment is performed. In addition, in order to further prevent such apractically undoped region forming near the bottom of the hard maskstructure 206, the doping concentration of the boro-silicate glass layer10 can be higher than that of the second boro-silicate glass layer 30.Here, the doping concentration of the boro-silicate glass layer 10 isabout 4×10¹⁷ to 8×10¹⁷ atom/cm² and that of the second boro-silicateglass layer 30 is about 1×10¹⁷ to 5×10¹⁷ atom/cm².

Next, in FIG. 5, a cross-section of the multi-layer hard mask structure206 according to another embodiment of the invention is shown. Themulti-layer hard mask structure 206 includes a first hard mask layer 205and a plurality of second hard mask layers 207 disposed thereon. In themulti-layer hard mask structure 206, the first hard mask layer 205 iscomposed of a boro-silicate glass layer 10 and an overlying undopedsilicon glass layer 20. The boro-silicate glass layer 10 has a thicknessof about 0.3 μm and the undoped silicon glass layer 20 has a thicknessof about 100 to 400 Å.

The second hard mask layer 207 is also composed of a plurality ofboro-silicate glass layers 30 and overlying undoped silicon glass layers40. Here, different to FIG. 4, all of the boro-silicate glass layers 30and the undoped silicon glass layers 40 have a total thickness of about1.2 to 1.4 μm and each undoped silicon glass layer 40 has a thickness ofabout 100 to 400 Å. Moreover, the doping concentration of theboro-silicate glass layer 10 is about 4×10¹⁷ to 8×10¹⁷ atom/cm² andsubstantially equal to that of each second boro-silicate glass layer 30.In the invention, the undoped silicon glass layers 20 and 40 are used asdiffusion barrier layers for double hard mask layers 205 and 207,respectively, to prevent a formation of a practically undoped regionnear the bottom of the hard mask structure 206 due to the poor dopingconcentration uniformity after heat treatment is performed. Moreover,the doping concentration uniformity of the entire hard mask structure206 can be further increased after the second boro-silicate glass layers30 are annealed. Also, in order to further prevent formation of such apractically undoped region near the bottom of the hard mask structure206, the doping concentration of the boro-silicate glass layer 10 can behigher than that of the second boro-silicate glass layer 30. Here, thedoping concentration of the boro-silicate glass layer 10 is about 4×10¹⁷to 8×10¹⁷ atom/cm² and that of each second boro-silicate glass layer 30has a doping concentration of about 1×10¹⁷ to 5×10¹⁷ atom/cm².

As mentioned above, in order to prevent deep trenches have variousdepths and critical dimensions, the hard mask structure 206 containingboro-silicate glass is necessary to be annealed. In the invention, theannealing is performed at 550 to 600° C. for 15 to 25 min.

Thereafter, a polysilicon layer 208 is formed overlying the multi-layerhard mask structure 206 by conventional deposition, such as CVD, at adeposition temperature of about 550 to 620° C. The polysilicon layer 208has a thickness of about 0.2 to 0.3 μm and serves as a hard mask todefine the multi-layer hard mask structure 206.

Next, a photoresist pattern layer 210 is formed overlying thepolysilicon layer 208 by lithography, which has at least one opening toexpose the region for deep trench etching. Thereafter, the polysiliconlayer 208 under the opening is etched by, for example, reactive ionetching (RIE), to form an opening 212 therein and expose a portion ofthe multi-layer hard mask structure 206.

Next, in FIG. 3 b, after the photoresist pattern layer 210 is removed byashing or suitable solution, the multi-layer hard mask structure 206 andthe underlying pad dielectric layer 202 and 204 are successively etchedusing the polysilicon layer 208 as an etch mask to form an opening 214therein and expose a portion of surface of the substrate 200.

Next, in FIG. 3 c, the exposed substrate 200 is etched by, for example,RIE, to simultaneously form a deep trench 216 therein and remove thepolysilicon layer 208. Meanwhile, a portion of the multi-layer hard maskstructure 206 is consumed due to etching.

Finally, the multi-layer hard mask structure 206 is removed by VHF toexpose the surface of the pad dielectric layer 202 and 204. Aspreviously mentioned, since there is no practically undoped regionformed near the bottom of the multi-layer hard mask structure 206, themulti-layer hard mask structure 206 can be completely removed, as shownin FIG. 3 d. Moreover, in the invention, since the multi-layer hard maskstructure 206 is removed by VHF, no undercutting occurs on the thinsilicon oxide layer 202.

Compared with the conventional method for etching a deep trench in asubstrate using a single boro-silicate glass layer as a hard mask, themulti-layer hard mask structure of the invention can improve the boronatom concentration uniformity and prevent the deep trenches from havingvarious depths and critical dimensions after annealing is performed onthe multi-layer hard mask structure. Moreover, in the multi-layer hardmask structure of the invention, an undoped silicon layer is disposedbetween each boro-silicate glass layer to serve as a diffusion barrierlayer, thereby preventing continuous upward diffusion of boron atoms.Accordingly, the hard mask structure can be removed completely, therebyincreasing device yield. Moreover, in the invention, a boro-silicateglass layer having higher doping concentration can be disposed in thebottom of the multi-layer hard mask structure, thereby furthermaintaining boron atom concentration uniformity.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation toencompass all such modifications and similar arrangements.

1. A multi-layer hard mask structure, comprising: a first hard masklayer composed of a first boro-silicate glass layer and an overlyingfirst undoped silicon glass layer; and at least one second hard masklayer disposed on the first hard mask layer, which is composed of asecond boro-silicate glass and an overlying second undoped silicon glasslayer.
 2. The hard mask structure as claimed in claim 1, wherein adoping concentration of the first boro-silicate glass layer issubstantially equal to that of the second boro-silicate glass layer. 3.The hard mask structure as claimed in claim 2, wherein the dopingconcentration of the first boro-silicate glass layer is about 4×10¹⁷ to8×10¹⁷ atom/cm².
 4. The hard mask structure as claimed in claim 2,wherein the first boro-silicate glass layer has a thickness of about 0.3μm.
 5. The hard mask structure as claimed in claim 1, wherein the firstundoped silicon glass layer has a thickness of about 100 to 400 Å. 6.The hard mask structure as claimed in claim 1, wherein the secondundoped silicon glass layer has a thickness of about 100 to 400 Å. 7.The hard mask structure as claimed in claim 1, wherein a dopingconcentration of the first boro-silicate glass layer is higher than thatof the second boro-silicate glass layer.
 8. The hard mask structure asclaimed in claim 7, wherein the doping concentration of the firstboro-silicate glass layer is about 4×10¹⁷ to 8×10¹⁷ atom/cm².
 9. Thehard mask structure as claimed in claim 7, wherein the dopingconcentration of the second boro-silicate glass layer is about 1×10¹⁷ to5×10¹⁷ atom/cm².
 10. A method for etching a deep trench in a substrate,comprising the steps of: forming a multi-layer hard mask structureoverlying the substrate, comprising a first hard mask layer and at leastone second hard mask layer disposed thereon, wherein the first hard masklayer is composed of a first boro-silicate glass layer and an overlyingfirst undoped silicon glass layer and the second hard mask layer iscomposed of a second boro-silicate glass layer and an overlying secondundoped silicon glass layer; forming a polysilicon layer overlying themulti-layer hard mask structure; etching the polysilicon layer to forman opening therein and expose a portion of the multi-layer hard maskstructure; successively etching the multi-layer hard mask structure andthe underlying substrate under the opening to simultaneously form thedeep trench in the substrate and remove the polysilicon layer; andremoving the multi-layer hard mask structure.
 11. The method as claimedin claim 10, further forming a pad oxide layer and an overlying siliconnitride layer between the substrate and the multi-layer hard maskstructure.
 12. The method as claimed in claim 10, further annealing themulti-layer hard mask structure before the polysilicon layer is formed.13. The method as claimed in claim 12, wherein the annealing isperformed at 550 to 600° C.
 14. The method as claimed in claim 12,wherein the annealing is performed for 15 to 25 minutes.
 15. The methodas claimed in claim 10, wherein a doping concentration of the firstboro-silicate glass layer is substantially equal to that of the secondboro-silicate glass layer.
 16. The method as claimed in claim 15,wherein the doping concentration of the first boro-silicate glass layeris about 4×10¹⁷ to 8×10¹⁷ atom/cm².
 17. The method as claimed in claim10, wherein the first boro-silicate glass layer has a thickness of about0.3 μm.
 18. The method as claimed in claim 10, wherein the first undopedsilicon glass layer has a thickness of about 100 to 400 Å.
 19. Themethod as claimed in claim 10, wherein the second undoped silicon glasslayer has a thickness of about 100 to 400 Å.
 20. The method as claimedin claim 10, wherein a doping concentration of the first boro-silicateglass layer is higher than that of the second boro-silicate glass layer.21. The method as claimed in claim 20, wherein the doping concentrationof the first boro-silicate glass layer is about 4×10¹⁷ to 8×10¹⁷atom/cm².
 22. The method as claimed in claim 20, wherein the dopingconcentration of the second boro-silicate glass layer is about 1×10¹⁷ to5×10¹⁷ atom/cm².
 23. The method as claimed in claim 10, wherein thepolysilicon layer has a thickness of about 0.2 to 0.3 μm.